Network Working Group H. Eidnes
Request for Comments: 2317 SINTEF RUNIT
BCP: 20 G. de Groot
Category: Best Current Practice Berkeley Software Design, Inc.
P. Vixie
Internet Software Consortium
March 1998
Classless IN-ADDR.ARPA delegation
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
2. Introduction
This document describes a way to do IN-ADDR.ARPA delegation on non-
octet boundaries for address spaces covering fewer than 256
addresses. The proposed method should thus remove one of the
objections to subnet on non-octet boundaries but perhaps more
significantly, make it possible to assign IP address space in smaller
chunks than 24-bit prefixes, without losing the ability to delegate
authority for the corresponding IN-ADDR.ARPA mappings. The proposed
method is fully compatible with the original DNS lookup mechanisms
specified in [1], i.e. there is no need to modify the lookup
algorithm used, and there should be no need to modify any software
which does DNS lookups.
The document also discusses some operational considerations to
provide some guidance in implementing this method.
3. Motivation
With the proliferation of classless routing technology, it has become
feasible to assign address space on non-octet boundaries. In case of
a very small organization with only a few hosts, assigning a full
24-bit prefix (what was traditionally referred to as a "class C
network number") often leads to inefficient address space
utilization.
Eidnes, et. al. Best Current Practice [Page 1]

RFC 2317 Classless IN-ADDR.ARPA delegation March 1998
One of the problems encountered when assigning a longer prefix (less
address space) is that it seems impossible for such an organization
to maintain its own reverse ("IN-ADDR.ARPA") zone autonomously. By
use of the reverse delegation method described below, the most
important objection to assignment of longer prefixes to unrelated
organizations can be removed.
Let us assume we have assigned the address spaces to three different
parties as follows:
192.0.2.0/25 to organization A
192.0.2.128/26 to organization B
192.0.2.192/26 to organization C
In the classical approach, this would lead to a single zone like
this:
$ORIGIN 2.0.192.in-addr.arpa.
;
1 PTR host1.A.domain.
2 PTR host2.A.domain.
3 PTR host3.A.domain.
;
129 PTR host1.B.domain.
130 PTR host2.B.domain.
131 PTR host3.B.domain.
;
193 PTR host1.C.domain.
194 PTR host2.C.domain.
195 PTR host3.C.domain.
The administration of this zone is problematic. Authority for this
zone can only be delegated once, and this usually translates into
"this zone can only be administered by one organization." The other
organizations with address space that corresponds to entries in this
zone would thus have to depend on another organization for their
address to name translation. With the proposed method, this
potential problem can be avoided.
4. Classless IN-ADDR.ARPA delegation
Since a single zone can only be delegated once, we need more points
to do delegation on to solve the problem above. These extra points
of delegation can be introduced by extending the IN-ADDR.ARPA tree
downwards, e.g. by using the first address or the first address and
the network mask length (as shown below) in the corresponding address
Eidnes, et. al. Best Current Practice [Page 2]

RFC 2317 Classless IN-ADDR.ARPA delegation March 1998
$ORIGIN 128/26.2.0.192.in-addr.arpa.
@ IN SOA ns.B.domain. hostmaster.B.domain. (...)
@ NS ns.B.domain.
@ NS some.other.name.server.too.
;
129 PTR host1.B.domain.
130 PTR host2.B.domain.
131 PTR host3.B.domain.
$ORIGIN 192/26.2.0.192.in-addr.arpa.
@ IN SOA ns.C.domain. hostmaster.C.domain. (...)
@ NS ns.C.domain.
@ NS some.other.third.name.server.
;
193 PTR host1.C.domain.
194 PTR host2.C.domain.
195 PTR host3.C.domain.
For each size-256 chunk split up using this method, there is a need
to install close to 256 CNAME records in the parent zone. Some
people might view this as ugly; we will not argue that particular
point. It is however quite easy to automatically generate the CNAME
resource records in the parent zone once and for all, if the way the
address space is partitioned is known.
The advantage of this approach over the other proposed approaches for
dealing with this problem is that there should be no need to modify
any already-deployed software. In particular, the lookup mechanism
in the DNS does not have to be modified to accommodate this splitting
of the responsibility for the IPv4 address to name translation on
"non-dot" boundaries. Furthermore, this technique has been in use
for several years in many installations, apparently with no ill
effects.
As usual, a resource record like
$ORIGIN 2.0.192.in-addr.arpa.
129 CNAME 129.128/26.2.0.192.in-addr.arpa.
can be convienently abbreviated to
$ORIGIN 2.0.192.in-addr.arpa.
129 CNAME 129.128/26
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RFC 2317 Classless IN-ADDR.ARPA delegation March 1998
Some DNS implementations are not kind to special characters in domain
names, e.g. the "/" used in the above examples. As [3] makes clear,
these are legal, though some might feel unsightly. Because these are
not host names the restriction of [2] does not apply. Modern clients
and servers have an option to act in the liberal and correct fashion.
The examples here use "/" because it was felt to be more visible and
pedantic reviewers felt that the 'these are not hostnames' argument
needed to be repeated. We advise you not to be so pedantic, and to
not precisely copy the above examples, e.g. substitute a more
conservative character, such as hyphen, for "/".
5. Operational considerations
This technique is intended to be used for delegating address spaces
covering fewer than 256 addresses. For delegations covering larger
blocks of addresses the traditional methods (multiple delegations)
can be used instead.
5.1 Recommended secondary name service
Some older versions of name server software will make no effort to
find and return the pointed-to name in CNAME records if the pointed-
to name is not already known locally as cached or as authoritative
data. This can cause some confusion in resolvers, as only the CNAME
record will be returned in the response. To avoid this problem it is
recommended that the authoritative name servers for the delegating
zone (the zone containing all the CNAME records) all run as slave
(secondary) name servers for the "child" zones delegated and pointed
into via the CNAME records.
5.2 Alternative naming conventions
As a result of this method, the location of the zone containing the
actual PTR records is no longer predefined. This gives flexibility
and some examples will be presented here.
An alternative to using the first address, or the first address and
the network mask length in the corresponding address space, to name
the new zones is to use some other (non-numeric) name. Thus it is
also possible to point to an entirely different part of the DNS tree
(i.e. outside of the IN-ADDR.ARPA tree). It would be necessary to
use one of these alternate methods if two organizations somehow
shared the same physical subnet (and corresponding IP address space)
with no "neat" alignment of the addresses, but still wanted to
administrate their own IN-ADDR.ARPA mappings.
Eidnes, et. al. Best Current Practice [Page 5]

RFC 2317 Classless IN-ADDR.ARPA delegation March 1998
The following short example shows how you can point out of the IN-
ADDR.ARPA tree:
$ORIGIN 2.0.192.in-addr.arpa.
@ IN SOA my-ns.my.domain. hostmaster.my.domain. (...)
; ...
1 CNAME 1.A.domain.
2 CNAME 2.A.domain.
; ...
129 CNAME 129.B.domain.
130 CNAME 130.B.domain.
;
$ORIGIN A.domain.
@ IN SOA my-ns.A.domain. hostmaster.A.domain. (...)
; ...
;
host1 A 192.0.2.1
1 PTR host1
;
host2 A 192.0.2.2
2 PTR host2
;
etc.
This way you can actually end up with the name->address and the
(pointed-to) address->name mapping data in the same zone file - some
may view this as an added bonus as no separate set of secondaries for
the reverse zone is required. Do however note that the traversal via
the IN-ADDR.ARPA tree will still be done, so the CNAME records
inserted there need to point in the right direction for this to work.
Sketched below is an alternative approach using the same solution:
$ORIGIN 2.0.192.in-addr.arpa.
@ SOA my-ns.my.domain. hostmaster.my.domain. (...)
; ...
1 CNAME 1.2.0.192.in-addr.A.domain.
2 CNAME 2.2.0.192.in-addr.A.domain.
$ORIGIN A.domain.
@ SOA my-ns.A.domain. hostmaster.A.domain. (...)
; ...
;
host1 A 192.0.2.1
1.2.0.192.in-addr PTR host1
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RFC 2317 Classless IN-ADDR.ARPA delegation March 1998
host2 A 192.0.2.2
2.2.0.192.in-addr PTR host2
It is clear that many possibilities exist which can be adapted to the
specific requirements of the situation at hand.
5.3 Other operational issues
Note that one cannot provide CNAME referrals twice for the same
address space, i.e. you cannot allocate a /25 prefix to one
organisation, and run IN-ADDR.ARPA this way, and then have the
organisation subnet the /25 into longer prefixes, and attempt to
employ the same technique to give each subnet control of its own
number space. This would result in a CNAME record pointing to a CNAME
record, which may be less robust overall.
Unfortunately, some old beta releases of the popular DNS name server
implementation BIND 4.9.3 had a bug which caused problems if a CNAME
record was encountered when a reverse lookup was made. The beta
releases involved have since been obsoleted, and this issue is
resolved in the released code. Some software manufacturers have
included the defective beta code in their product. In the few cases
we know of, patches from the manufacturers are available or planned
to replace the obsolete beta code involved.
6. Security Considerations
With this scheme, the "leaf sites" will need to rely on one more site
running their DNS name service correctly than they would be if they
had a /24 allocation of their own, and this may add an extra
component which will need to work for reliable name resolution.
Other than that, the authors are not aware of any additional security
issues introduced by this mechanism.
7. Conclusion
The suggested scheme gives more flexibility in delegating authority
in the IN-ADDR.ARPA domain, thus making it possible to assign address
space more efficiently without losing the ability to delegate the DNS
authority over the corresponding address to name mappings.
8. Acknowledgments
Glen A. Herrmannsfeldt described this trick on comp.protocols.tcp-
ip.domains some time ago. Alan Barrett and Sam Wilson provided
valuable comments on the newsgroup.
Eidnes, et. al. Best Current Practice [Page 7]

RFC 2317 Classless IN-ADDR.ARPA delegation March 199811. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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Eidnes, et. al. Best Current Practice [Page 10]